1. Field of the Invention
The present invention relates to a semiconductor device including a driver to drive a control signal of a switching element.
2. Description of Related Art
A relay which has been used in an engine control unit etc. as an automotive product is recently replaced by a semiconductor device such as a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) with no contact point. Further, an IPD (Intelligent Power Device) which includes a current limiter, an overheat detector and so on in addition to a power MOSFET is recently employed.
FIG. 5 illustrates an example of an IPD according to a related art. The IPD 900 of FIG. 5 has a withstand output voltage of 60V and can be driven at 5V. As shown in FIG. 5, the IPD 900 includes an output MOS transistor M90 which is connected between an OUT terminal and a GND terminal. The gate of the output MOS transistor M90 is connected to an IN terminal. The IPD 900 is driven according to a control signal of a microcomputer. A 5V drive signal S0 is input to the IN terminal from the microcomputer. The IPD 900 further includes a protector 91 having a current limiter, an overheat detector and so on which is connected between the IN terminal and the GND terminal to thereby protect the output MOS transistor M90 from overcurrent and overheating.
A load (L-load) containing inductance is connected to the OUT terminal. The load is driven by turning on/off the output MOS transistor M90. Because the load is connected between a power supply and a switch so that the switch is located on the low-voltage side of the load, the IPD serves as a low-side switch in this configuration.
FIG. 6 illustrates another example of an IPD according to a related art. The IPD 901 of FIG. 6 has a withstand output voltage of 100V or higher. In the IPD 901 with such a high withstand output voltage, the output MOS transistor M90 has degraded on-resistance characteristics, and it is necessary to drive the output MOS transistor M90 with a highest possible gate voltage. Therefore, the IPD 901 includes a level shifter 92 which converts the drive signal S0 of 5V into a gate signal S1 of 14V at a battery voltage level.
In the level shifter 92, a resistor R91 and a MOS transistor N91 connected in series and a resistor R92 and a MOS transistor N92 also connected in series are connected in parallel between a Vcc terminal and a GND terminal. The level shifter 92 shifts the level of the drive signal S0 input to the IN terminal to the level of the voltage supplied to the Vcc terminal, and outputs the level-shifted gate signal S1 to the gate of the output MOS transistor M90.
FIGS. 7A to 7C illustrate waveforms where no voltage is applied to the Vcc terminal in the IPD 901. FIG. 7A shows the voltage at the OUT terminal, FIG. 7B shows the gate voltage at the output MOS transistor M90, and FIG. 7C shows the current at the OUT terminal.
In the IPD 901, when no voltage is applied to the Vcc terminal, the gate signal S1 is not output from the level shifter 92, and the gate voltage at the output MOS transistor M90 is 0V. If a steep voltage rise occurs to cause a step-like voltage change at the OUT terminal in time t1 as shown in FIG. 7A, the gate voltage of the output MOS transistor M90 increases due to parasitic capacitors C91 and C92 of the output MOS transistor M90 as shown in FIG. 7B. This causes the output MOS transistor M90 to temporally turn on, which causes a current to flow into the OUT terminal as shown in FIG. 7C.
The IPD 901 of a related art has a drawback that the steep voltage rise occurring at the OUT terminal results in a current flow to the OUT terminal.
As a technique to overcome such a drawback, Japanese Unexamined Patent Application Publication No. 2002-185299 (Seki) discloses a semiconductor device to reduce a through current of an output transistor. This semiconductor device increases the output impedance of an output driver when no power is supplied to a pre-driver to thereby prevent a through current from flowing into the output driver.
FIG. 8 illustrates a case where the semiconductor device with push-pull structure taught by Seki is applied to a low-side IPD with open-drain structure.
In the IPD 902 of FIG. 8, the output of a pre-driver 911 is shifted by a level shifter 912 and then inverted by an inverter 918 to thereby turn on/off an output MOS transistor 920 and drive an L-load 919. The IPD 902 also includes a power supply voltage detector 930 to detect the voltage at a power supply terminal LVDD.
The power supply voltage detector 930 cuts off the output MOS transistor 920 when no voltage is supplied to the power supply terminal LVDD of the pre-driver 911. Specifically, when a voltage is not supplied to the power supply terminal LVDD, an Nch MOS transistor 915 of the power supply voltage detector 930 is off. Then, an inverter 916 outputs a Low level according to the voltage between a power supply terminal HVDD and the L-load 919, a Pch MOS transistor 917 thereby turns on, and the inverter 918 outputs a Low level. The output MOS transistor 920 is thereby cutoff (turns off).
In the IPD 902, however, when no voltage is supplied to the power supply terminal HVDD, the power supply voltage detector 930 does not operate and the output of the inverter 918 is indeterminate. In the state where the power supply voltage is low and the inverter is unstable, if a voltage is applied to the OUT terminal through the L-load 919, the gate voltage of the output MOS transistor 920 increases to cause the output MOS transistor 920 to turn on, thus failing to overcome the drawback that a current flows to the L-load 919.
As described in the foregoing, in the semiconductor device such as the IPD of a related art, a steep, step-like voltage rise occurring at an OUT terminal causes an increase in the gate voltage of the OUT terminal due to parasitic capacitors of an output MOS transistor, which undesirably results in a current flowing into the OUT terminal.
There is a case where supply of power is controlled through separate power switches even when the power supply connected to an OUT terminal and a Vcc terminal is the same battery voltage. In such a case, due to a time lag between turn-on timing of the respective power switches, a voltage can be applied to the OUT terminal through an L-load when no voltage is applied to a level shifter, thus causing the above drawback to occur.
An L-load connected to an IPD is typically a coil for turn on/off an electromagnetic relay or a coil used in an injector to inject fuel to an engine. A current flowing into such a coil causes erroneous operation of the electromagnetic relay or the injector, and this is very serious problem in terms of reliability as a component which is used for an engine control unit as an automotive product, which should place ultimate priority on the safety.